1. Field of the Invention
This invention relates to an objective lens driving apparatus used for recording and reproducing data in respect of an information recording medium such as an optical disk.
2. Description of the Related Art
As is well known, optical disk devices are widely employed in which data is stored and retrieved using a laser beam. Typical examples are compact disk device (CD) and laser disk device (LD). Recently, optical disk devices have come to be employed as storage devices for computers.
When information is recorded onto or reproduced from an optical disk, a laser beam is focused onto the optical disk surface by an objective lens to form a spot. The smaller the size of this spot, the greater the recording density of the optical disk that can be achieved. As technology advances, recording density increases. Therefore, new technology such as DVD, requires ever smaller spot sizes.
Furthermore, to maintain the size of the spot as small as possible, it is important to exercise control such that the optical axis of the objective lens is as near as possible perpendicular to the optical disk surface, so as to produce a condition in which little coma aberration can occur. Coma aberration may occur if the objective lens is out of alignment, for example, or if the optical disk is out of alignment because of, for example, warping.
A known objective lens drive construction whereby such control can be achieved involves supporting the objective lens for example by means of four wires. Such an objective lens driving device is constructed so as to make possible not only displacement of the focusing direction and the tracking direction but also axial rotary displacement for correcting a tilt of the optical axis of the objective lens, by means of focusing coils and tracking coils arranged at the periphery of the objective lens.
However, in order to correct a tilt of the optical axis of the objective lens in high accuracy, desired biaxial rotary displacement should occur. Furthermore, large variations in wire shape result in imbalance of the drive characteristic, making it very difficult to achieve the desired operating characteristic. Thus the device ends up becoming impracticable.